Various methods are known for determining the concentration of a gas component of a measurement gas. They include direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS).
In direct absorption spectroscopy, the wavelength of a laser is varied via a current ramp and the detector signal is recorded. Light is absorbed by the measurement gas in accordance with the Beer-Lambert law when running through the current ramp when the laser passes through the region of the absorption line.I=I0*e−α(λ)cL 
α(λ) is the absorption dependent on the wavelength;
c is the gas concentration;
and L is the path over which the gas absorbs.
The detector signal is thereby deformed such that in comparison with the detector signal without gas absorption an absorption line can be recognized which can then e.g. be fit by a fit with a physical model function (a Voigt absorption line as a rule) and the absorption surface (surface below the absorption curve) can be determined which is proportional to the gas concentration.
Further evaluation processes of direct absorption spectroscopy are also known in which, for example, the surface below the absorption line is determined without physical model functions.
Wavelength modulation spectrometry (WMS) is a form of optical absorption spectroscopy which makes possible a detection of very small optical absorptions because the absorption measurements of small frequencies (near DC) at which the light sources have high noise are shifted toward high frequencies in which shot noise is the limiting factor. This frequency shift can improve the measurement sensitivity by three to five orders of magnitude.
WMS is carried out as a rule using continuously tunable lasers such as diode lasers (TDL). In this respect, the wavelength is tuned slowly over an absorption line of the measurement gas and is additionally lightly modulated by a modulation frequency f, typically sinusoidal, with is high in comparison. When the light beam which is wavelength-modulated in this manner propagates through the measurement path, an amplitude modulation of the light results from the intensity change of the laser and by the absorption of the measurement gas. When the light is then detected in the light receiver and a received signal is produced in dependence on time, the received signal includes AC components at the modulation frequency f and its harmonics 2f, 3f, 4f, etc. One of the AC components can be selected for the evaluation and can be evaluated in a phase-sensitive process, e.g. using a lock-in process. This process is also called demodulation. The signal obtained at the frequency nf on a demodulation is called an nf signal (n=1, 2, 3, . . . ). The demodulated signal thus includes information with respect to the optical absorption and the intensity of the light beam. Concentrations of a gas component of the examined measurement gas can be determined via the absorption measured in this manner.
A detailed theory which describes WMS and the relationships between the shape of the absorption line and the shape of the demodulated signal is given in “Frequency-modulation spectroscopy for trace species detection: theory and comparison among experimental methods,” Applied Optics 31, 707-717 (1992). The signal shape which is obtained in WMS when the absorption line is moved over slowly and the wavelength is simultaneously modulated at the frequency nf corresponds in quality to the nth derivation of the absorption line so that the name derivative absorption spectroscopy is also used for WMS.
A method is known from DE 102 38 356 A1 in which the two measurement processes WMS and DAS are used alternately in consecutive periods and the detected signals are likewise alternately supplied to two separate averaging processes and evaluated. In the WMS evaluation, the results of the DAS evaluation can be used, e.g. for calibration. The calibration freedom of direct absorption spectroscopy and the accuracy of wavelength modulation spectroscopy are thus obtained.
In a method known from U.S. Pat. No. 7,616,316 B2, a switch is made between DAS at high concentrations of the gas component to be measured and WMS at low concentrations thereof. The measurement method which appears the most suitable is therefore respectively used.
Both measurement methods, WMS and DAS, are used simultaneously in DE 10 2012 223 874 B3 or are used alternately as in DE 102 38 356 A1 and the two measurement results are linked by averaging, whereby smaller errors can be achieved in the result.
The invention has the objective of increasing the functional safety which can be defined with the aid of the so-called safety-integrated level (SIL) in gas concentration determinations to be able to satisfy safety standards which define a specific SIL level. It is possible that an error tolerance of 1 is required in the functional safety. This normally relates to the hardware. Independently of the hardware used, non-random errors can, however, occur in the gas concentration measurement due to external influences such as intensity fluctuations, pressure changes, pressure, temperature and interference. It is therefore the object of the invention to provide a method improved with respect to the functional safety and spectrometers for determining a gas component.
Such an increase in safety could be achieved by using two different measurement methods, DAS and WMS, as known from DE 102 38 356 A1 and DE 10 2012 223 874 B3. It is, however, disadvantageous that this means a doubling of the effort with respect to time and apparatus. In the simultaneous application of the measurement methods disclosed in DE 10 2012 223 874 B3, the laser has to be controlled using a ramp plus sinusoidal modulation. This is disadvantageous for DAS for the direct detector signal in WMS is not suitable for an evaluation according to DAS since the sinusoidal modulation not only produces a power modulation, but also a wavelength modulation. The absorption is also scattered by the sinusoidal modulation after an averaging over e.g. a sinusoidal period and the evaluation is difficult and can no longer be achieved via a fit using a physical model. The result of such a DAS is therefore not as significant since the laser is controlled in an unfavorable manner for this purpose.